The End-Triassic mass extinction occurred approximately 201 million years ago and is associated with massive volcanic eruptions associated with the breakup of Pangea. Those eruptions released huge amounts of CO2 into the atmosphere, raising global temperatures by an estimated 5 to 10 degrees Celsius.
As the planet warmed, tree-bearing forests were destroyed. Ferns rapidly moved across degraded landscapes, spreading across large parts of what is now northwestern Europe and creating extensive savanna-like environments. New research by an international team led by geologists from Utrecht University shows that these fern-covered areas were highly vulnerable to fire. The fern itself may have supplied most of the fuel that kept the fire spreading.
The findings were published in nature geology On 21 July 2026.
Reconstruction of ancient wildfires
To investigate wildfire activity from this distant period, researchers studied exceptionally well-preserved sediments from 4 drill cores. One of them was a 640-meter-long core recently collected from the United Kingdom.
The team reconstructed ancient fire activity by measuring fossil charcoal and organic compounds, known as polycyclic aromatic hydrocarbons (PAHs), produced in wildfire smoke.
When these results were combined with the record of fossil pollen and spores, they pointed to a sharp increase in wildfire activity during the main phase of the extinction. This fiery interlude also matches the dramatic expansion of the fern.
However, both traditional indicators have limitations. Large coal pieces may break into many smaller pieces, making the size of the fire appear larger than it actually is. PAHs can travel far from the fire that produced them, and some molecules may not survive in the geological record. Because of these problems, researchers developed another method to track fires in deep time.
“The novelty of this study comes from the analysis of color changes of organic microfossils,” explains the paper’s senior author, Dr. Bas van de Schootbrugge of Utrecht University. “We used a simple and very low-cost technique that measures the ‘darkness’ of fossil pollen and spores, a so-called palynomorph darkness index.”
A strange pattern in fossil color
Organic microfossils usually become darker after burial as the material gradually changes with increasing pressure and temperature. Sediments that go deeper into the ground are exposed to more heat, causing the organic matter inside them to cook faster. In most cases, greater depth means deeper fossils.
“But here we found a very different pattern,” says Van de Schoutbrugghe.
The oldest and darkest pollen and spores in the core remained light colored. Yet through the extinction interval the fossils became progressively darker, eventually reaching extremely dark brown. Once the extinction period ended, the fossils returned to pale yellow.
“We were quite surprised by this phenomenon because it occurs in all 4 cores at exactly the same time, so it cannot be related to the burial of sediments as the four basins have experienced very different geological histories,” explains Van de Schoutbrugge.
Ancient Fire “Dark Zone”
The Palynomorph Darkness Index measures color using the RGB spectrum. A camera attached to the light microscope records the fossils, and the color information is converted to an average gray scale value. This allows scientists to compare samples from different layers within the same core, as well as samples taken from cores in different locations.
The researchers completed 15,000 measurements of pollen and spores from surviving plants before, during, and after the extinction. They compared tree pollen to fern spores to determine whether the darkness could be due to biological differences between the plant groups.
“All plant groups show similar effects, which is a strong indication that this was the result of some external force.”
When the team compared the fossil’s color changes to the charcoal and PAH levels, the pattern became clear. The unusual “dark zone” appeared to record an extended period of severe wildfire activity during the fern spike.
“The blackening exactly overlaps with the fern spike, the main extinction interval, and the increased abundances of charcoal and PAHs.”
Ferns spread across a warming world
The rapid rise of ferns during the main extinction interval was likely driven by several linked forces, including deforestation, soil erosion, intense greenhouse warming, and frequent wildfires.
Van de Schoutbrugge: “Ferns are truly remarkable plants that have faced many crises in Earth’s history, and some species can adapt to some of the most extreme environments. They can be considered true disaster species.”
Some ferns can spread rapidly on damaged ground, especially where other vegetation has been destroyed. Fire can accelerate this process. Although the visible parts of the fern burn, the plants can rapidly regrow from root systems below the surface. This allows them to return faster and take over even more area than many competing plants.
That ability may help explain why fern spikes have lasted so long. Researchers estimate that this interval continued for at least 40,000 years and possibly as long as 300,000 years.
Fern became fuel for recurring fires
“When the ferns dry out, the thick mats act as ideal fuel to fuel large-scale wildfires,” explains Van de Schootbrugge.
Rapidly spreading pioneer and weeding ferns created extensive fern savannas. Some species may have acted as fire ladders, helping flames advance through the landscape, while also helping to crowd out and destroy other vegetation.
“The ferns responded and delivered fuel that fanned the flames, causing massive wildfires again and again. A hellish world indeed.”
This can result in a destructive feedback loop. Climate warming and deforestation have opened up the landscape to ferns. The ferns provided abundant dry fuel for new fires, after which they rapidly grew back and spread again.
“The lesson we can learn from this is that climate change, deforestation and the spread of opportunistic species could provide all the ingredients for a perfect storm,” Van de Schootbrugge concluded.